I think it's a waste of time to try to equate ancient currency to modern ones. There's something to be gained from it I guess, but generally I just ignore attempts to do so, they're not very helpful or very accurate. Whatever we use to estimate the modern value of ancient currency, the method is always lacking. The two main methods are to either use the value of the corresponding weight of whatever precious metal the coin was made out of, or to look at the price of grain and go from their. Both are, in my opinion, next to useless. The ancient economy was totally unlike our own. The Green Revolution has drastically reduced the price of all food, the Industrial Revolution has brought entirely new industries to the front of the economy while creating a consumer market that would not have involved any such goods in such quantities. Perhaps most important, if we're looking at daily wages (which is used occasionally to estimate modern values for ancient currency), the way that jobs work in the modern world is 100% different. Modern societies are dominated by either salaried or hourly jobs. Neither would have been known in antiquity. The idea of an hourly job is an invention of the Industrial Revolution--workers who didn't own shops in antiquity were generally day-workers, living a pretty hand-to-mouth existence. Now, Cicero claims that even an unskilled slave could find work for three denarii (around three drachmae--this is four hundred years after the estimates given on Wikipedia, and it's among the urban population at Rome, not Athens) a day, but his statement is rarely taken at face value. Three denarii is not much, but there's enough reason to suppose that Cicero is being too gracious to be skeptical--grain at 6 1/3 asses per modius was too expensive, and Tacitus says that at the beginning of Tiberius' reign praetorian guardsmen were still being paid two asses a day (Caesar had raised the pay of the regular legions to 10 asses, at which value it remained during this time). Still, let's take Cicero's word for it. Problems immediately arise when we do so. For one thing, this was not a steady wage--a day-laborer might be paid more or less depending on what he was doing, and he might not find work at all for that day. Another issue is what he was purchasing. The wage of a day-laborer in the Roman city was little more than a survival wage--with it he had to pay his absurdly expensive rent (often daily) and purchase food and clothing for himself and his family. Even considering that in many families all members must have provided labor, the income of a Roman laborer and his family was woeful. Now, if we try to equate this to a modern value, we run into the problem that nothing that not only is this laborer's work environment totally unknown to the vast majority of members of the modern economy, but the stuff that he's purchasing is being sold in a very different market. Take grain, for example, the most fundamental purchase of the ancient world. Prior to the Green Revolution grain's value was at a premium. I can go down to the store and buy a loaf of bread for like five bucks--this was not so in antiquity, even assuming a stable price of grain (which was rare).

Anyway, in other words I'm saying that you're very right to say that converting ancient currency to modern requires all kinds of assumptions, about the value of commodities, about the way labor works, about the entire economy really. The best way--as far as I'm concerned the only way--to really understand the value of currency in antiquity is to become familiar with the texts and get an idea of what was being bought and sold for how much, and how money moved around--sadly this is frequently extremely difficult with the Roman urban poor, as most of our sources on their financial situations are very distant and don't even necessarily know what they're talking about